Resolve "Mirroring for external CI/CD repositories should not store OAuth token"

What does this MR do?

Oauth2 tokens are causing issues with mirroring repos, because it effectively limits the number of repos you can mirror. Personal Access Tokens do not have this problem.

This change removes the OAuth2 option from the import page, and only provides the personal access token form.

It also brings the form up to current design standards.

Normal import with OAuth configured

Before	After

Normal import without OAuth configured (and I'm an admin)

Before	After

CI/CD only import

Before	After

Related to #59053 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers